A newcomer, recently neutered, will fight with your resident cats. It is as much about working out who is in charge as anything else. The one with the most hormones is most likely to be wiling to have a fight - especially a former stray boy. It takes a while for the hormones to work their way out of a cat's system. and it also takes a while to introduce an adult cat into an existing household. Unless there is actual bloodshed, I would let them get on with it with minimum intervention.,..